This invention relates to a method for producing a printed circuit board; and, more particularly, to a method of producing a printed circuit board having wirings on both sides of the insulating board, in which part of the wiring passes over holes in the insulating board.
Conventionally, printed circuit boards comprise a film-like insulating board and a conductor (wiring) pattern printed on the insulating board. Some of them have a conductor pattern on each side of the insulating board. The conductor patterns on both sides of the insulating board are electrically interconnected by means of via-elements and plated through-holes (PTH) formed along via-holes and through-holes in the insulating board.
Some of such double-sided printed circuit boards contain, for example, holes different from the via-holes and the through-holes on the insulating board, and part of the conductor patterns on them pass over so as to bridge the holes in the insulating boards. In the following description, holes to be used as the via-elements and the plated through-holes are termed primary holes, and the other holes are termed secondary holes.
Portions of said conductor patterns that pass over the secondary holes of the insulating board are, for example, are electrically connected to external pins of a semiconductor chip to be mounted on the printed circuit board. In the following description, the portions that pass over holes in the printed circuit board are termed conductor bridges.
One method of manufacturing double-sided printed circuit boards having such conductor bridges comprises, for example, the steps of forming a conductor pattern on the first main side of the insulating board, forming holes in the insulating board by laser etching, and forming a conductor pattern on the side (the second main side) opposite to the first main side of the insulating board. (Japanese Application Patent Laid-Open Publication No. Hei 10-12677 (FIG. 1 to FIG. 3; hereinafter, Patent Document 1)
In summary, the method of manufacturing printed circuit boards as disclosed in Patent Document 1 comprises the steps of bonding a conductor film, such as a copper foil, to each side of an insulating board, such as a polyimide sheet, forming via-elements and plated through-holes in the insulating board to electrically interconnect the conductor films on both sides of the insulating board, patterning one of the conductor films to form a first conductor pattern on the insulating board, applying laser beams, such as from a carbon dioxide laser, onto the first conductor pattern layer to etch and form secondary holes (device holes) in the insulating board, and then patterning the other conductor film to form a second conductor pattern having conductor bridges.
Besides the above-described laser-etching method disclosed in Patent Document 1, the secondary holes (device holes) can be formed on the insulating board also by an alkaline etching process, which uses an alkaline solution. However, the types and dimensions of insulating boards to which the alkaline etching is applicable are limited. Further, alkaline etching has some disadvantages, such as low dimensional stability and low shape accuracy of the holes, and it uses a highly-toxic etching solution.
However, although the above-described method can solve problems that the alkaline etching has by forming secondary holes (device holes) in the insulating board by laser etching, it takes a lot of time to etch and form big holes, such as device holes.
Further, in the conventional method of manufacturing printed circuit boards, a long processing time, a high maintenance cost (on a laser oscillator), and a resulting high manufacturing cost can not be avoided.